238 HOST PENETRATION 



icitv of filtrates from cultures in which 5. rolfsii had been grown 

 became greater with increase in oxalic acid content. For these 

 reasons Higgins stated that the evidence appears conclusive in 

 showing that oxalic acid, secreted by fungus hvphae, causes death 

 of cells in advance of actual penetration. 



Other pertinent evidence was presented by Brown (1915) from 

 his experiments with extracts from germ tubes of Botrytis cinerea. 

 These extracts are highly active in decomposing parenchymatous 

 tissues of manv kinds of vegetables and fruits. Heating to 60° to 

 70° C inactivated this extract, and he was unable to separate 

 enzyme from toxic principle. Drops of extract, when placed on 

 delicate rose petals, were quite innocuous, provided that the cu- 

 ticle was intact. Brown found no oxalic acid in the extract and 

 was forced to conclude as follows: (a) that the only active con- 

 stituent of the extract was pectinase, and (b) that he had failed 

 to extract a toxic principle, leaving unfounded the killing in ad- 

 vance of penetration described by de Bary. 



In early stages of invasion bv Diplocarpon rosae, browning of 

 the host cells has been observed [Aronescu (1934)1, but the im- 

 mediate cause has not been determined. 



When Blackman and Welsford (1916) and Boyle (1921) made 

 cytoloeical examination of tissues attacked bv Botrytis cinerea 

 and by Sclerotinia libertiana, thev noted that the staining reactions 

 of the host cell beneath the germ tube were early modified and 

 that a very slender "infection hvpha" always penetrated the cells 

 in advance of killing-. Once this had been established, Brown 

 (1922) determined bv conductivity tests that rapid exosmosis of 

 solutes from the tissues does not occur unless the infection hvphae 

 have penetrated, and thus he was able to establish with some de- 

 gree of finality how facultative parasites are able to attack host 

 cells. 



The role of appressoria. Certain pathogenic fungi, notably of 

 the genera Colletotrichum, Gloeosporium, and Marssonia, form pe- 

 culiar structures called appressoria, which function in penetration 

 of the suscept. Frank (1883) first recognized the true nature of 

 appressoria in connection with observations on Fusicladium trevi- 

 ulae, Poly stigma nibrnm, and Colletotrichum lindeinuthiamnn. 

 He interpreted them to be adhesion disks which applied them- 

 selves closely to the surface to be penetrated and there served to 

 anchor the pathogen while the membranes immediately beneath 



